Download the electromagnetic wave

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The signal radiates radially outwards from our source dipole and we must now consider how it disperses.
We can consider our source of radio frequency signal as a point source. The true radiated wavefront is then spherical but
because we view it at a distance from the source we can regard it as a plane wave. However, due to this spherical spreading,
the wave becomes weaker as it travels. The intensity of the electric and magnetic fields of the signal each vary inversely with
the distance they travel. Travel twice the distance and each of these has dropped to half.
This “inverse distance law” is related to another law, the “inverse square law” which is a power consideration. The power
passing through a spherical surface is the same as that passing through a spherical surface at twice the distance but at twice the
distance it is distributed over four times the surface area, so the power-per-unit-of-area there is one-quarter.
When dealing with waves, we usually consider their "strength" or "intensity" as the electric or magnetic field strength, or the
voltage induced in an aerial, and with these words we mean their amplitude. Similarly, when we consider two separate waves
at a point, we are usually interested in the sum of their amplitudes, the sum of the amplitudes of their electric fields.
A changing electric field induces an adjacent changing magnetic field, which in turn induces another adjacent changing
electric field, then another adjacent changing magnetic field… We now have a transverse electromagnetic wave… energy in
the changing electric and magnetic fields moving away … at the speed of light.
Points to note:
A CHANGING electric field will induce a CHANGING magnetic field in the surrounding region and vice-versa. The
changing fields in the surrounding region will, in turn, induce further fields in a still more distant region, so the energy
continues to propagate on its journey outwards.
An electromagnetic wave cannot exist unless it is moving. Once created, it will continue on forever unless it is absorbed by
matter.
Electromagnetic waves require no material medium to support them. They propagate just as well in a completely empty space,
in a vacuum, as in the atmosphere.
At the receiver:
A dipole aerial at a receiver can act as the recipient of energy from the wave. A wave arriving at a receiver, an electromagnetic
wave passing by a conductor, induces a current in that conductor.
Our simple diode-type field-strength-meter receiver indicates the electrons being shifted between the two sections of the dipole
by the passing wave. It shows the resulting signal from the sum of all the waves present and cannot separate out energy being
received from individual waves. It has a current induced in it from all the passing waves. It contains a rectifier which rectifies
the resulting signal current to produce a half-wave-rectified direct current which deflects the meter. The meter responds to only
the positive-going half-waves of each component signal and this unidirectional current is a relative indication of the intensity
of the resulting wave.